1. Field of the Invention
The invention relates to concrete finishing trowels which employ one or more rotatable blade-equipped rotor assemblies for finishing a concrete surface. More particularly, the invention relates to a concrete finishing trowel, such as a riding trowel, incorporating a torque transfer system for the rotor assembly or assemblies that has a variable speed ratio and that accommodates tilting of at least the driven shaft of the rotor assembly during a steering operation.
2. Description of the Related Art
A variety of machines are available for smoothing or otherwise finishing wet concrete. These machines range from simple hand trowels, to walk-behind finishing trowels, to self-propelled finishing trowels including some larger walk-behind machines as well as relatively large two-rotor or even three-rotor machines. Self-propelled finishing trowels, and particularly riding finishing trowels, can finish large sections of concrete more rapidly and efficiently than manually pushed finishing trowels. The invention is directed to self-propelled finishing trowels and is described primarily in conjunction with riding finishing trowels by way of explanation.
Riding concrete finishing trowels typically include a mobile frame including a deck. At least two, and sometimes three or more, rotor assemblies are mounted on an underside of the deck. Each rotor assembly includes a driven shaft extending downwardly from the deck and a plurality of trowel blades mounted on and extending radially outwardly from the bottom end of the driven shaft and supported on the surface to be finished. The driven shafts of the rotor assemblies are driven by one or more self-contained engines mounted on the frame and typically linked to the driven shafts by gearboxes of the respective rotor assemblies. The weight of the finishing trowel and the operator is transmitted frictionally to the concrete by the rotating blades, thereby smoothing the concrete surface. The individual blades usually can be tilted relative to their supports, via operation of a suitable mechanical lever and linkage system accessible by an operator seated on an operator's platform to alter the pitch of the blades, and thereby to alter the pressure applied to the surface to be finished by the weight of the machine. This blade pitch adjustment permits the finishing characteristics of the machine to be adjusted. For instance, in an ideal finishing operation, the operator first performs an initial "floating" operation in which the blades are operated at low speeds (on the order of about 30 rpm) but at high torque. Then, the concrete is allowed to cure for another 15 minutes to one-half hour, and the machine is operated at progressively increasing speeds and progressively increasing blade pitches up to the performance of a finishing or "burning" operation at the highest possible speed--preferably above about 150 rpm and up to about 200 rpm.
The blades of riding trowels can also be tilted, independently of pitch control for finishing purposes, for steering purposes. By tilting the driven shafts of the rotor assemblies, the operator can cause the forces imposed on the concrete surface by the rotating blades to propel the vehicle in a direction extending perpendicularly to the direction of driven shaft tilt. Specifically, tilting at least the driven shaft of the rotor assembly from side-to-side and fore-and-aft steers the vehicle in the forward/reverse and the left/right directions, respectively. It has been discovered that, in the case of a riding trowel having two rotor assemblies, the driven shafts of both rotor assemblies should be tilted for forward/reverse steering control, whereas only the driven shaft of one of the rotor assemblies needs to be tilted for left/right steering control.
The rotor assemblies of the typical riding finishing trowel are driven by a drive train that is connected directly to input shafts of the assemblies' gearboxes via a centrifugal clutch and a system of shafts, belts or chains, and other torque transfer elements of constant speed ratio. The drive trains also require universal joints to accommodate tilting of the gearbox relative to the remainder of the drive train during a steering control operation. The universal joints are expensive to maintain and must be maintained or replaced relatively frequently due to the ingress of concrete into the universal joints and their attendant bearings.
Another problem associated with traditional rotor assembly drive systems is that they exhibit an insufficient speed range for both low speed/high torque floating operations and high speed burning operations. The typical drive system includes a simple centrifugal clutch of a constant speed ratio. Hence, blade speed increases at least generally proportionately with engine speed from zero to a maximum speed, with torque decreasing commensurately over that same engine speed range. No known concrete finishing trowel has a constant speed ratio clutch that can obtain both the necessary low speed/high torque combination required for optimal floating operations and the high speed required for optimal burning operations. Hence, many contractors keep two machines at each job site--one having a relatively low speed ratio and configured for floating operations, and one having a relatively high speed ratio and configured for burning operations. This requirement significantly increases the expense of a particular finishing operation.
The above-identified problems associated with drive systems having traditional centrifugal clutches can be alleviated if the traditional centrifugal clutch is replaced with a hydrostatic drive system, as is the case in the HTS-Series Ride on Power Trowel marketed by Whiteman Corp. of Carson, Calif. However, hydrostatic drive systems still exhibit a less than optimal speed/torque range. They are also relatively expensive and heavy when compared to more traditional, mechanical-clutch operated drive systems. The hydraulic components of these hydrostatic systems are also prone to failure and leakage.
Applicants are aware of one attempt to alleviate these problems by using a variable speed ratio torque converter assembly to transfer torque from the engine to the rotor assemblies of a riding concrete finishing trowel. Specifically, Bartell Corp. proposed the use of a torque converter assembly to permit the speed ratio of a concrete finishing trowel's rotor assemblies to change during the operation of the machine. The torque converter assembly included drive and driven variable-speed clutches that operated in conjunction with one another so that, as the engine accelerated, the relative diameters of the sheaves of the drive and driven clutches changed to increase the machine's speed ratio as the engine speed increased. However, testing revealed that the clutches of this torque converter assembly were improperly sized and configured. As a result, the desired effect of providing a single machine capable of operating at low rpm and high torque and high rpm and low torque was not achieved.